1. Field of the Invention
The present invention relates to a solar cell module excelling especially in moisture resistance and transparency. More particularly, the present invention relates to a solar cell module improved so that the solar cell characteristics are effectively prevented from being deteriorated due to a reduction in the shunt resistance and the like of the photovoltaic element when used under environmental conditions with high temperature and high humidity over a long period of time.
2. Related Background Art
Recently, a number of solar cell modules have been proposed. FIG. 1 is a schematic cross-sectional view illustrating the constitution of a typical example of these solar cell modules. In FIG. 1, reference numeral 1101 indicates a photovoltaic element (or a solar cell) having a collecting electrode 1108, reference numeral 1102 a surface side filler, reference numeral 1103 a surface protective layer (or film), reference numeral 1105 a back side filler, reference numeral 1106 a insulating member, and reference numeral 1107 a support member (or a back face reinforcing member). Particularly, the surface protective layer 1103 comprises a fluororesin film such as an ethylene-tetrafluoroethylene copolymer (ETFE) film or polyvinyl fluoride (PVF) film; the surface side filler 1102 comprises ethylene vinyl acetate copolymer (EVA) or butyral resin; the back side filler 1105 comprises EVA (which is the same as the surface side filler 1102) or ethylene-ethyl acrylate copolymer (EEA); and the insulating member 1106 comprises a film of an organic resin such as nylon or polyethylene terephthalate (PET) or a member comprising an aluminum foil sandwiched with Tedlar (trademark name). In this solar cell module, the surface side filler 1102 serves also as an adhesive between the photovoltaic element 1101 and the fluororesin film as the surface protective layer 1103, and the back side filler 1105 serves also as an adhesive between the photovoltaic element 1101 and the insulating member 1106. The fluororesin film as the surface protective layer 1103 together with the surface side filler 1102 serve to prevent the photovoltaic element 1101 from being eternally damaged and from being damaged from external shock. The insulating member 1106 is disposed in order to reinforce the solar cell module while adding an appropriate rigidity thereto.
The collecting electrode 1108 of the photovoltaic element is usually formed by using a metallic wire coated by an electrically conductive composition or by way of screen printing of an electrically conductive paste.
In such solar cell module, EVA is usually used as the surface side filler 1102. And in order to sufficiently enclose the photovoltaic element 1101, a crosslinking agent such as 2,5-dimethyl-2,5-bis(t-butylperoxy)hexane (one-hour half life temperature: 138.degree. C.) is incorporated into the EVA as the surface side filler. Besides this, it is known to use a peroxide compound capable of being decomposed at low temperature as the crosslinking agent for the EVA, where the EVA is crosslinked by way of decomposition of said peroxide compound at low temperature. In the case of using said peroxide compound as the crosslinking agent for the EVA as the surface side filler, in the lamination process for producing a solar cell module, the crosslinking of the EVA as the surface side filler proceeds at a high speed (this will be hereinafter referred to as high speed EVA crosslinking manner) and therefore, the heat treatment in the lamination process can be accomplished in a short period of time, resulting in reducing the period of time required for the lamination process. The use of the high speed EVA crosslinking manner provides other advantages since the heat treatment in the lamination process can be accomplished for a short period of time as above described, the quantity of heat energy applied to covering materials including the EVA as the surface side filler and a fluororesin film as the surface protective film in the heat treatment is relatively small so that the covering materials are prevented from being yellowed due to the heat energy applied. Therefore, the formation of a surface side cover excelling in optical initial characteristics can be attained for the photovoltaic element.
In the case of a solar cell module having the foregoing surface side cover excelling in optical initial characteristics formed by way of the high speed EVA crosslinking process, the fluororesin film which is present in the surface side cover as the surface protective film situated on the outermost surface side has a satisfactory water repelling effect for preventing moisture damage but it is difficult to attain a satisfactory moisture barrier function by the fluororesin film only. In addition, the photovoltaic element is sealed by the EVA having a high water absorbability which is situated under the fluororesin film. Because of this, the solar cell module is not sufficiently stable for the long-term case where the solar cell module is continuously used under environmental conditions with high temperature and high humidity. Further, in the case where the collecting electrode of the photovoltaic element comprises a metallic wire coated by an electrically conductive composition comprising particles of an electrically conductive material and a binder resin, the coat of the metallic wire is unavoidably accompanied by gaps present among the electrically conductive particles The gaps are left by insufficient filling with the binder resin. The metallic wire is therefore not sufficiently protected from contact with moisture.
Now, the high speed EVA crosslinking process is advantageous in that the EVA can be crosslinked in a short period of time, but it exhibits a problem in that the period of time during which the EVA is maintained in a fluidized state is short. Because of this, irregularities present at the photovoltaic element and the gaps present at the collecting electrode comprising the metallic wire coated by the electrically conductive composition are not sufficiently filled by the EVA and such irregularities and gaps provide unfilled defects in the solar cell module. This situation is liable to cause such problems as will be described as follows. When moisture invades the solar cell module, the moisture passes through said unfilled defects to reach the metallic wire of the collecting electrode. In this case, the metallic wire is oxidized to cause an increased series resistance (Rs) or/and the metal of the surface of the metallic wire is ionized or/and precipitated, when the photovoltaic element is in a voltage applied state. The ionized or precipitated metal migrates to deposits in the defects of the photovoltaic element, thus resulting in short circuits (or shunts) in the photovoltaic element. These deteriorate the photoelectric conversion performance of the solar cell module particularly when the solar cell module is continuously used under severe environmental conditions of high temperature and high humidity over a long period of time.
Further, for the collecting electrode comprising the metallic wire coated by the electrically conductive composition, when the moisture invaded the electrically conductive composition as above described, there is a tendency that the adhesion between the collecting electrode and the photovoltaic element gradually becomes inferior to provide an increased contact resistance between them, and the electric power generated by the photovoltaic element cannot be efficiently utilized over a long period of time.
Even in the case where the collecting electrode is formed by way of screen printing of an electrically conductive metal paste, the collecting electrode formed of the metal paste is liable to have gaps as well as in the case of the collecting electrode comprising the metallic wire coated by the electrically conductive composition. Therefore, there is a tendency that when moisture invades the solar cell module, there is a tendency that problems similar to the foregoing problems in the case of the collecting electrode comprising the metallic wire coated by the electrically conductive composition occur in that the metallic material of the collecting electrode is ionized or/and precipitated, and wherein the metal ionized or precipitated migrates to deposit in the defects of the photovoltaic element, resulting in causing short circuits (or shunts) in the photovoltaic element.
In addition, in the case where a glass fiber member is contained in the surface side filler comprising the EVA in order to make the surface side cover have an improved scratch resistance, a problem is liable to occur in that moisture often invades through the interface between the glass fiber member and the surface side filler, and it is difficult to sufficiently protect the photovoltaic element from moisture invasion.
Now, in the case of using EVA and a conventional crosslinking agent in combination as the surface side filler in the production of a solar cell module, since the period of time during which the EVA is crosslinked by the crosslinking agent is made to be relatively long, the EVA is maintained in a fluidized state for a relatively long period of time. Therefore, the irregularities present at the photovoltaic element and the gaps present in the collecting electrode are possible to be sufficiently filled by the EVA. However, if the gaps present in the collecting electrode could be sufficiently filled by the EVA, there is a tendency that since the EVA itself has a high water absorbability as above described, it is difficult to prevent collecting electrode suffering from moisture invasion. Hence, it is difficult to attain a solar cell module having an improved moisture resistance. Besides this, when the EVA by which the gaps of the collecting electrode are filled, contracts with the metallic wire of the collecting electrode, there is a tendency that the EVA is yellowed which creates a problem in that the quantity of light absorbed in the photovotaic element is diminished to cause a reduction in the photoelectric conversion efficiency provided by the photovoltaic element.
Further, in order to attain a solar cell module having an improved moisture resistance, there are a number of proposals to use a glass member as the outermost surface covering member of the solar cell module. According to these proposals, to use the glass member as the outermost surface covering member of the solar cell module makes it possible to prevent moisture invasion into the solar cell module from the surface side but it is difficult to sufficiently prevent moisture invasion into the solar cell module from the side faces thereof. In order to prevent the moisture invasion from the side faces of the solar cell module, there is known a manner of sealing the side faces of the solar cell module by means of a silicone sealant. However, the sealed side faces employing silicone sealant are poor in long-term moisture resistance and moisture once penetrating into the solar cell module barely released outside. The solar cell module having the outermost surface cover comprising the glass member, inferior in flexibility and shock resistance, and is weighty and costly.
In order to attain a solar cell module having an improved moisture resistance, there is known a manner of making the surface protective layer comprising an organic resin film such as a fluororesin film having an improved moisture resistance by depositing a film of SiO.sub.2, SiO.sub.x or an alumina on at least one of the opposite surface of the organic film by means of the CVD sputtering process. However, the film deposited on the organic film as the surface protective layer is often colored so that it is poor in transparency and because of this, the resulting solar cell module is inferior in initial characteristics. Further in this case, the film deposited on the organic film as the surface protective layer is usually highly crystalline and therefore it is hard. Hence, the flexibility which is a representative feature of the film module is diminished, and the film deposited on the organic film as the surface protective layer is liable to crack to allow moisture invasion when the module is excessively bent. Therefore, this is not always effective in improving the moisture resistance of the solar cell module.
Now, a solar cell module is often used by installing it on a roof of a building. In this case, in order for the solar cell module to be used in a given country, the solar cell module must meet the requirements prescribed in the standard relating to roofing materials in that country. As one of the requirements, there is a combustion test. In order to pass the combustion test, the amount of the EVA belonging to a combustible resin which is used in the solar cell module as the sealing material must be decreased. However, in the case where the amount of the EVA used in such solar cell module as above described is decreased, the protective ability of the surface side cover to protect the photovoltaic element is diminished accordingly. In order to solve this problem, there is a proposal of reinforcing the EVA by means of a glass fiber member. In this proposal, there is employed a manner of disposing the glass fiber member in the surface side cover so that the surface side cover has an ability of protecting the photovoltaic element. In this case, it is necessary to use the EVA in such an amount that the glass fiber member can be sufficiently packed in the surface side cover. However, the solar cell module having such a surface side covering configuration is difficult to be approved that it is a roofing material belonging to Class A in the combustion test prescribed in UL 1703 Standard of the U.S.A.